Electronic semiconductor packages are well-known and are configured in several different ways. Typically, an electronic semiconductor package includes: a silicon chip (die) containing circuit elements; a substrate, for example, a printed circuit board (PCB); first level interconnects which connect the die and the substrate, i.e., wirebonds, Tape Automated Bonds (TAB) and Controlled Collapse Chip Connection (C4 or flip chip bonds); and second level interconnects, such as external metal pins or solder balls, which connect the substrate to printed wiring circuit cards. Substrates comprise ceramic or plastic materials depending on the particular application. Some semiconductor packages have encapsulant which coats the die and the first level interconnects for protection. A dam or stiffener ring may also be used to hold the encapsulant in place around the die and interconnects as it hardens. Finally, a lid covers the top of the package to protect and dissipate heat from the die and first level interconnects.
A cross-sectional view of a typical cavity-up electronic semiconductor package is shown in FIG. 1. A semiconductor chip or die 1 is attached to a substrate 2 by die attach epoxy 1a. The die 1 electrically communicates with the traces 3 of the substrate 2 by bond wires 4. Encapsulant 6 resides over the die 1 and wirebonds 4. A layer of epoxy 7 is spread over the encapsulant 6 and a lid 8 is attached thereto by the epoxy 7. Solder balls 9 attach the substrate 2 to a printed wiring circuit card, not shown.
During operation, energy is lost in the form of heat which builds up in the electronic semiconductor packages. As junction temperatures increase, reliability and speed are sacrificed. Because failure processes accelerate with increased temperature, the life of each junction becomes shorter as the temperature increases. It is generally well-known to provide means for dissipating heat from the electronic semiconductor package. It is particularly important to dissipate heat from electronic packages with semiconductor devices which operate at high speeds and high power levels.
The path of heat dissipation is considered in two parts: junction-to-case path and case-to-ambient path. The junction-to-case path usually conducts heat directly from the die to a package surface through an encapsulant epoxy. Then the heat is convected from the semiconductor package case to a surrounding ambient (case-to-ambient path) either by natural convection or forced convection. The case-to-ambient heat transfer is enhanced by increasing surface area exposed to ambient air. This is accomplished by employing heat sinks which have many configurations. Fluids such as air, water, etc. are passed over the heat sink to exchange heat from the heat sink surface to the fluid or ambient.
Because the heat dissipation effectiveness of a heat dissipation device is dependent on the thermal resistance of the path between the junction and the case, the means of attaching the lid to the die and substrate is critically important. Typically, lids are constructed from a high thermal conductivity material, such as copper, aluminum or high thermal conductivity plastic. They are attached to the packages with thermally conductive adhesive or epoxy, such as "LOCTITE-384" produced by Loctite Corp.
Typically, the lids are attached to packages by hand. A large drop of epoxy is placed on the top of the encapsulant of the glob top package. The lid is then placed on top of the glob top and pressed against the package to squeeze the epoxy between the lid and the encapsulant. The entire package is then placed in an oven for curing.
This process, however, results in misaligned lids and poor conduction paths between the dies and the lids. In the first instance, the lids may be placed on the packages off center and tilted prior to the curing process. Further, as the packages are cured, both the encapsulant of the glob tops and the epoxy between the lids and the glob tops soften and deform which causes the lids to shift. In fact, the lids may be off target, relative to center, by as much as +/-0.050 inches and the lids may not rest flat, relative to the substrate.
Therefore, there is a need for a method and device for attaching lids to packages which does not allow the lids to drift from center or to tilt to one side of the package during the attachment and curing processes.